Process for the preparation of aliphatic alpha-unsubstituted alpha-beta-unsatureated carboxylic acids and their derivatives



pllo, l957 A. wou-RAM ET AL 2,790,822

PROCESS FOR THE PREPARATION oF ALIPHATICLUNSUBSTITUTED L-p-UNSATURATEDCARBOXYLIC ACIDS AND THEIR DERIVATIVES Filed Aug. 10. 1954A INVENTORnerf/ ae WOL Fen/14 A. HEM/Z srf/L ATTORNEY nit States Patent() PROCESSFR THE PREPARATION OF ALIPHATIC ia-UNSUBS'EITUTED a--UNSATURATED CAR-BOXYLHC ACIDS AND THEIR DERIVATIVES Arthur Wolfram, rankfurt am Main,Karl-Heinz Steil, Knapsack, near Koln, and Artur Agunte, Lechenich,lnear Koln, Germany, assignors vto Knapsack-GriesileirnAktiengesellschaft, Knapsack, near Koln, Germany Application August10,1954, Serial'No. 453,656 Claimspriority, application ,Germany August12, 1953 9 Claims. y(Cl. v260-465.9)

It is known that 1t-substituted.a-hydroxy-carboxylic acids or theirderivatives can be converted into the corresponding a--unsaturatedcompounds. nt-Methacrylic acid nitrile was obtained by the actionOf-phosphorus ,pentoxide'on acetone cyanhydrin and -ot-phenyl-acrylicacid (atropic acid) was obtained from a-phenylfa-hydroxypropionic acid(atrolactinic acid) by boiling with hydrochloric acid. n

When trying to splitoff water-from lactic acid, there are obtainedlactide, lactyl lacticacid, and other compounds. .According to,another.method, lacticacid splits over pumice at 440460 C.intoacetaldehydeandformic acid, and lactic acid nitrile did not furnishany acrylonitrile either when reacted withphosphorus pentOxide.

Up to now, it-was, therefore, necessary to prepare the especiallyimportant, wunsubstituted af/S-unsaturated acids and their derivativesby indirect ways. In ordertoobtain the technically` importantacrylonitrile, for instance, from the easily accessible lactic `'acidnitrile, the hydroxy group ofthe lactic acid nitrile-was, for instance,acetylated with acetic anhydride and a molecule of acetic acid was ,thensplit off at a raised temperature. kAccording to another process, thepreparation of Vacrylonitrile-from lactic acid nitrile was dispensedwith and, instead. of ,this step, the more expensivelEt-hydroxy-propionic.acid nitrile Was used as starting materialfor themanufacture of acrylonitrile.

We have found that m38-unsaturatedaliphatic carboxylic acids which areunsubstituted in a-position can be obtained from the correspondinghydroxy-acids if these are treated at temperatures ranging from 520 C.to 700 C., preferably from 580 C. to 650 C.,` for a short time withacidic agents.

Not only the acids can be prepared according to the process of thepresent invention but alsotheirderivatives, especially the nitriles oramides kfromthe corresponding derivatives of thehydroxyfcarboxylicacids. The process according to the presentinventionis especially ysuitable for the manufacture ofthea--unsaturated carboxylic acids with 3 to 5 carbon atoms and,particularlyv for acrylonitrile. The carbon chain can alsobe branchedin-position. According to theprocess 'ofthevpresent invention, it is alsopossible, however,'to prepare oci-substituted. aunsaturated carboxylicacids and. their derivatives, for instance methacrylieacid `anditsderivatives. k According to the invention, there mayv generally bevprepared compounds of the formula l (I) R1 H inl whichRi andiRzrepresent hydrogenV and saturated alkyl .and preferably contain notemorethan'2 Vcarbon yatoms and Rs stands'for ice R4 and R5 representhydrogen, methyl or ethyl. Asstarting material there are used compoundsof the formula Rz OH in which R1, R2 and R3 have the same significancevas in Formula l and R3 can, in addition, stand for the Agroup --.COOR6,Re standing preferably for alkyl with `1-4 carbon atoms. According tothe process of ythepresent invention there vcan be obtained, forinstance, acrylic acid, crotonic acid, ethyl acrylic acid, andfdiinethyl acrylic acid as Well as theirv nitriles, furthermoretheunsubstituted amides, the iN-methylfsubstituted amides, theN-dimethyl, N-,ethyl-, ,Nfethylfmethylg andj,N-diethyl substitutedamides ofthe compounds `quoted abovlfur therrnore carboxylic acids with;adongercarbon chain, such as v-propyl acrylic acidfbutyl acrylic.,acid,,diethyl acrylic acid, ,B-methyI-.-propyl acrylic acid etc., and theirderivatives.

lAs acidic agentsthere may be usedphosphoric acid,

pyrophosphcric acid, metaand polyphosphoric acids containing not morethan ppho'sphorus atoms, hydrogen Vchloride and mixtures lof theseacids. nPhosphoric acid has` proved to beparticularly effective. Theuse` of phosphoric acids is yalso of. advantage in `so far as theycan,easily be separated from the` reaction products on account of theirnon-volatility.

For the reaction according to the present invention there ,are used l-90percent,y preferably 20,-40l percent ,of the inorganic acid, calculatedupon the` mixture. vInstead ofthe said mixtures, there ,may alsobey usedmixtures. 0f the acids mentioned above, alvantageou'slyv of phosphoricacid, with sulfuricvacid, acetic acid and. propionieacid. The Vabovementioned acids, for instance, l'thephosphoric acid, should be containedin the starting mixture .in axcon:

.centration ofat least 1 percent.

Theseparated phosphoric acid4 can immediatelybe re used if the correctconcentration was observed. It ,c onrtainsabout 10 percent ofpolyphosphoric acids, among them thepyrophosphoric acid. Whenrepeatedlyusingr this mixtureof phosphoric acids, `the abovequotedc'ompositiondoes notvvary. anymore, the reason being that the steam which ispresentat the Sametime constantly causes hydrolysis of the phosphoricacids. n Howeventhe yield of a-,G-unsaturated compounds, such asacrylonitrile, is not inuenced unfavorably if the phosphoric acid usedfor dehydration contains portions of polyphosphoric acids. y

It is'also possible, although less advantageousogusepolyphosphoricacids.containing not more` than 6 phosphorus atoms. Inthishcase, however, partial hydrolysis constantlytakes place atthe hightemperaturesbymeans Vof thevwvater. .Poly/phosphoric acids Lin admixturewith Water have the same, effect as orthophosphoric acid.

The above mentioned acids can` advantageously 1be used `in a Qhydrousform. fif, howeverhydrogenchloride vis used, the anhydrous acid may also4be employed.

It isin the riatureofvthe reaction that the functional terminal groupsof they acid .derivatives can likewise be altered by the acidic agentsused.` `Acrylic acid is-thus obtained from ethyllactate when usingphosphoric acid. Furthermore, when using hydrogen chloride, Atheacrylonitrile is/hydrolyzed during the'reactionA to acrylicacid Theprocess. of the present invention is advantageously carried, out underreduced pressure. lt is `Vof .advantage if theperiod under reducedpressure ranges from 0.1.1006

v second but ,the operationcanalsobecarried outw ith shorterpe'riodsor,in the presence ofV inert gases, `alsoiwith longer periods. Swuch `shortperiods` are, for Ainstance produced bypassing the lstartinglmaterialsunder .reduced pressure through the reaction tube.

In addition, the vapours of the compounds to be reacted can be dilutedwith larger quantities of indifferent gases which are free from oxygen,such as nitrogen or oxygen-free steam. Y

When working under reduced pressure, the presence of indifferent gases,for instance of steam, has likewise proved to be of advantage.

The acidic agents can also be applied on carrier substances, which areadvantageously installed in the reaction tube. It is not necessary toemploy porous carrier substances but carriers with small surfaces arepreferred. In the presence of phosphoric acid, carbon can only beemployed at low temperatures since, otherwise, a reduction of thephosphoric acid takes place. There are espe ciallysuitable quartz,natural calcium phosphate or aluminium phosphate, furthermorekieselguhr, and articial xerogels. If the acids are applied on carriersubstances they nced not be injected.

The yield can beraised considerably if the mixture consisting ofa-hydroxy-carboxylic acids or their derivatives and of acidic, liquidagents is introduced, in a finely divided state, into a heated reactiontower at temperatures ranging from S-700 C., preferably from S80-650 C.,in which case it is of advantage to operate under reduced pressure andto add water to the mixture or to blow steam into the tower.

The compound to be converted, for instance lactic acid nitrile, is mixedunder a pressure of about 30 to 100 mm, of mercury, with agents showingan acid reaction, for instance, with concentrated phosphoric acid, andthe reaction mixture is used in a nely divided state. This linedistribution can, for instance, be achieved by atomisation throughnozzles, that is to say, by spraying the reaction mixture as line aspossible through a nozzle into the chamber where the dehydration is totake place. The chamber which might be used with or withoutinstallations has a cylindrical shape and carries the injection nozzleat the top, so that the injected material can pass through the reactionchamber in a free fall and in a finely distributed state for instance,in the form of a spray. In the lower part of the reaction chamber whichis conical, the material is cooled. as rapidly as possible either bycooling from outside or by injection of water, consequently in thepresence of steam. The heat which is necessary for the reaction can besupplied by external heating or by exposing the spray cone of thereaction mixture directly to the action of indierent gases free fromoxygen, for instance, burnt gases which are free from oxygen and havethe required temperature.

Under certain conditions, especially, when the reaction heat is suppliedfrom outside and the fine distribution is etected by using inert gasesas carriers for the finely dis tributed reaction mixture, the reactionchamber can be provided with installations which enlarge the reactionsurfaces. ture is achieved by pressing it through nozzles, while heatingsimultaneously from outside, it is of advantage if the diameter of thereaction chamber is of such a size that the spray cone does not touchthe walls. In many cases it has proved to be lof advantage to mix themixture to be reacted directly with superheated steam in an eddycurrentnozzle.

The reaction mixture can be worked up in a simple manner. When operatingunder reduced pressure, the water and non-volatile acids, for instancephosphoric acid, are separated from the reaction products in aseparator. The phosphoric acid can, for instance, be recovered bydistillation and can then be re-utilised in admixture with, forinstance, lactic acid nitrile. The gases leaving the separator can bewashed once more for a short time in a spray tower with a dilutesolution of the acid used as starting material in order to remove thelast traces of this acid. When using a nitrile as starting material, thegases pass into a tower which is charged with a concentrated, weaklyalkaline solution ofthe same nitrile If the tine distribution of thereaction mixi (pH=7-8). Hydrocyanic acid and aldehyde combine to thenext higher nitrile which is re-introduced into the furnace. Theacrylonitrile, for instance, can then be prepared in known manner bywashing with water befor or after the vacuum pump.

lf, however, the operation is carried out under normal pressure in thepresence of burnt gases, the gases leaving the furnace and the liquidcan be completely cooled and the gases and the aqueous condensate canthen be worked up separately. When preparing acrylonitrile, the gaseousportion is, for instance, freed from hydrocyanic acid and acetaldehydein a spray tower charged with a weakly alkaline solution of lactic acidnitrile (pH 7-8) as described above. The acrylonitrile contained in thegas is removed in the manner described above. The homologous nitriles,the respective carboxylic acids, and the derivatives obtained by the twomethods can be worked up in the same manner.

The aqueous liquid is best evaporated under reduced pressure. At the topof the fractionating column used for this operation, hydrocyanic acid,acetaldehyde, and acrylonitrile are withdrawn and passed into the towersprayed with lactic acid nitrile. In the centre of the column water isobtained which may serve for washing the acrylontrile out of the gascurrent. Concentrated phosphoric acid is obtained as non-volatileportion which may be re-utilized as starting material. This process canbe rendered more economic from an energy point of view if thecondensation heat of the products leaving the furnace is used forheating the described vacuum column.

An apparatus for carrying out the process-which, in this case, concernsthe preparation of acrylonitrile--by means of pressing the materialthrough a nozzle is diagrammatically illustrated in the accompanyingdrawings l and 2.

In Figure 1 the vessel 1 contains the mixture consisting of phosphoricacid and lactic acid nitrile. By means of pressure nitrogen 11, themixture is forced through the tube 2 into the electric furnace 3 andthrough filter 4 into nozzle 5. The product passes the reaction tube 6with the lower conical part 7, then the cooler 8, and reaches iinallythe separator 9. Through the connecting piece 10 nozzle 5 may be fedwith steam.

Figure 2 shows the furnace which is directly heated with burnt gases.The burnt gases which are free from oxygen are produced in thecombustion chamber 12. They are prepared from the heating gas enteringat 13 and from air which is necessary for the combustion and enters at14. The steam required for adjustment of the reaction temperature isadmitted at 1S. The gases are conducted through the distributor ring 16into the reaction chamber 17. By means of pressure nitrogen 11, themixture of phosphoric acid and lactic acid nitrile is pressed from thevessel 1 through the pipe 2 into the furnace 18. It is filtered in 4 andfinely distributed by means of nozzle 19. Steam may also be suppliedthrough the connecting piece 20. The reaction product is cooled in thecooler 8 and collected in separator 9.

Although it has already been described to prepare crotonic acid nitrilefrom propionic aldehyde cyanhydrin (a-hydroxy-butyric acid nitrile) byheating with phosphorus pentoxide, it is expressly pointed out that thelactic acid nitrile CN-CH(OH)-CH3 does not yield the expected acrylicacid nitrile.

In this reaction, phosphorus pentoxide is only used as dehydrating agentwhich must always be renewed. In the process of the present inventionthe free acids are used. These acids have not only a catalytic elect butthey can also be used with advantage in the presence of water or in theform of the hydrous acids whilst in the reaction formerly describedwater must not be present.

The following examples serve to illustrate the invention, but they arenot intended to limit it thereto, the parts being by weight.

Example I (A) Through a quartz tube of 30 mm. internal diameter and 1000mm. length filled with pieces of quartz, there are passed, at 600 C. andunder a pressure of 20 mm. of mercury and for a duration of stay of 0.28second, 100 grams per hour of a mixture consisting of 100 parts oflactic acid nitrile, 70 parts of water, and 40 parts of phosphoric acidof 85 percent strength. The phosphoric acid is separated at 100 C. fromthe product leaving the reaction tube. Subsequently, the volatileconstituents are completely liquefied and worked up by distillation. 30percent of the lactic acid nitrile used as starting material areobtained as a mixture of acetaldehyde and hydrocyanic acid, 5l percentas acrylonitrile and 16 percent as lactic acid nitrile. The loss ofsubstance amounts to 3 percent.

(B) Through the apparatus described under A there are conducted, at thesame temperature and at a pressure of 760 mm. of mercury, 100 grams perhour of a mixture consisting of 100 parts of lactic acid nitrile, 1000parts of water, and 30 parts of phosphoric acid of 85 percent strength,the duration of stay amounting to 6.9 seconds. The product is worked upas described under A and yields percent of acrylonitrile in addition tounchanged lactic acid nitrile.

Example Z Through the apparatus described in Example 1, there arepassed, at a temperature of 600 C. and under a pressure of mm. ofmercury, 100 grams per hour of a mixture consisting of 100 parts ofa-hydroxy-butyric acid nitrile (obtained by addition of hydrocyanic acidto propionic aldehyde), 50 parts of water and 50 parts of phosphoricacid of 85 percentstrength. The duration of stay of the reaction mixturein the reaction tube amounts to 0.34 second. The volatile constituentsare completely condensed after leaving the furnace and are separatedfrom each other by distillation. There are obtained 45 percent ofcrotonic acid nitrile and 20 percent of a mixture of propionic aldehydeand hydrocyanic acid, and 18 percent of the unchanged starting material.The mixture of propionic aldehyde and hydrocyanic acid is then workedup-to a-hydroxy-butyric acid nitrile and re-introduced into the process.The entire yield amounts to about 83 percent.

Example 3 Through the apparatus described in Example 1 there are passed,at a temperature of 590 C. and under a pressure of 20 mm. of mercury,100 grams per hour of a mixture consisting of 100 parts of-hydroxy-valeric acid nitrile, 70 parts of glacial acetic acid, and 50parts of phosphoric acid of 85 percent strength. The duration of stayamounts to 0.58 second. The phosphoric acid is separated in theseparator. The volatile constituents are condensed in the otherreceivers cooled to 80 C., and worked up by distillation. 40 percent ofthe a-hydroxy- Valeric acid nitrile used as starting material areobtained in the form of -ethyl-acrylic acid nitrile in addition to 30percent of unchanged starting material. The whole yield amounts to about70 percent.

Example 4 After removing the quartz filling, there are passed throughthe quartz tube described in Example 1, at a temperature of 600 C. andunder a pressure of 20 mrn. of mercury, 100 grains per hour of a mixtureconsisting of 100 parts of lactic acid nitrile, 30 parts of water, and25 parts of hydrochloric acid of 36 percent strength. The duration ofstay amounts to 0.26 second. After 5 hours the reaction products areworked up `according to the method specified in Example 1. 5 percent ofthe lactic acid nitrile used as starting material is obtained in theform of acrylonitrile and 65 percent in the form of an aqueous solutionof acrylic acid.

It, instead of parts of hydrochloric acid of 36 percent strength, 30parts of aqueous hydrobromic acid of 30 percent strength are used,acrylonitrile and, especially, acrylic acid are formed.

Example 5 Through the quartz tube mentioned in Example 1 there arepassed, at a temperature of 620 C. and under a pressure of'20 mm. ofmercury, 90 grams per hour of a mixture of 100 parts of lactic acidethyl ester, 40 parts of water, and 30 parts of phosphoric acid of 85percent strength. The duration of stay amounts to 0.41 second. Theseparation of the phosphoric acid and the condensa.- tion of thereaction gases are carried out as described in Example l. The working upof the condensate by distillation shows that 55 percent of the esterused as starting material are converted into acrylic acid. In addition,acetaldehyde, ethyl alcohol and ethylene can be separated.

Example 6 Through the quartz tube mentioned in Example 1, there arepassed, at a temperature of 600 C. and under a pressure of 20 min. ofmercury, 100 grams per hour of a mixture consisting of 100 parts oflactic acid, 30 parts oi' water, and 20 parts of phosphoric acid of 85percent strength. The duration of stay amounts to 0.47 second. From theaqueous condensate obtained upon cooling of the reaction gases theacrylic acid formedwas separated by extraction with ether in a yield of40 percent.

Example 7 In an empty tube 6 (see Figure l) of refined steel which has alength of about 2000 mm. and an internal diameter of 230 mm. and isheated electrically to a wall temperature of 650 C., there are atomised3 kilograms per hour of a mixture consisting of 2 parts of lactic acidnitrile and 1 part of phosphoric acid of 85 percent strength under apressure of about 100 mm. of mercury by means of an eddy-current nozzle(5) run per hour with l kilogram of steam at 250 C. The duration of stayof the starting materials in the reaction tube amounts to 4.6 seconds.From the gases leaving the reaction tube the phosphoric acid isseparated at 100 C. Subsequently, the gases still under reduced pressureare cooled to 20 C. in an ascending cooler with a separator arrangedbelow it. The residual gases consisting of acrylonitrile, hydrocyanicacid, and acetaldehyde freed in the above manner from phosphoric acidand steam pass then a washing tower charged with phosphoric acid of 20percent strength V for elimination of any phosphoric acid vapours thatmight be carried along. Finally they reach a tower which is filled withRaschig rings and is sprayed with a weakly alkaline solution or percentstrength of lactic acid nitrile having a pH value of 7.0-7.5. Thepassing gaseous acrylonitrile is here freed from hydrocyanic acid andacetaldehyde by formation of lactic aci-d nitrile. The acrylonitrileleaving this tower passes now into the vacuum pump and is condensed onthe pressure side of the pump. Allowing for the quantity of the productwhich remains in the washing towers on account or" its solubility, thereare obtained, per hour, 1.10 kilogram of an acrylonitrile of percentstrength which contains small amounts of water, hydrocyanic acid,acetaldehyde, and lactic acid nitrile, which corresponds to a yield of70 percent. 0.46 kilo-gram (=23 percent) or" the lactic acid nitrile`decomposes in the reaction furnace to yield hydrocyanic acid andacetaldehyde which are again transformed into lactic acid nitrile. Theeffective yield of acrylonitrile amounts to about 90 percent. rEhephosphoric acid accumulating in the separator which is Worked at C. ismixed with new lactic acid nitrile for re-use in the furnace.

Example 8 The apparatus illustrated in Figure 2 is used for thisexample. Per hour l5 Nm3 of illuminating gas are completely burnt with56 Nm3 of air in the combustion chamber 12 which has an internal widthof 340 mm. and is lined with rebricks. The superheated burnt gases whichare free from oxygen are cooled to 1150 C. by mixing with steam of about1809 C. With this temperature the mixture of burnt gases and steampasses through a distributor ring 16 into a reaction furnace 18 which islined with iirebricks and has an internal width of 500 mm. and a lengthof 2000 mm. 43.2 kilograms per hour of a. mixture consisting of 2 partsof lactic acid nitrile and 1 part by weight of phosphoric acid of 85percent strength are supplied just above the distributor ring 16 by wayof 2, 4, and 19 in the manner as described in Example 7. The duration ofstay amounts to 3.1 seconds. The temperature prevailing at the furnaceentrance amounts to GOO-620 C. Upon leaving the furnace, the reactiongases are cooled by directly injecting water. The condensate is thenfreed from all reaction products by distillation. ln this manner thereare obtained per hour kilograms of acrylonitrile corresponding to 70percent of the lactic acid nitrile used as starting material inadmixture with 6.6 kilograms of hydrocyanic acid and acetaldehyde and2.4 kilograms of water. By working up this mixture of hydrocyanic acidand acetaldehyde to yield lactic acid nitrile and by leading the nitrileback into the process, a total yield of about 89 percent is obtained.

This example shows that the duration of stay of 0.1 to 0.6 second quotedabove need not be strictly observed and may be exceeded when operatingwith indiierent gases having a diluting effect.

From the above examples can be seen that the total yield can beconsiderably raised by the ne distribution of the reaction mixture.

We claim:

l. A process for preparing a compound of the formula:

wherein R1 and R2 are selected from the group consisting of hydrogen andalkylJ R1 and R2 together containing not more than two carbon atoms, R3is selected from the group consisting of -COOII,

-COOH,

-CON/ CN :and -COOR6, Rs being a lower alkyl radical of from yone tofour carbon atoms and R1, Rz, R4 and R5 having the values defined above,with an acid selected from the group consisting of phosphoric acid,pyrophosphoric acid, metaand polyphosphoric acids containing not morethan 6 phosphorous atoms and hydrochloric acid ata temperature of fromabout 520 700 C., the amount of acid by weight being from 1-90% of thetotal weight 'of the reactants.

2. The process of claim 1 wherein the reaction is carried out by heatingunder reduced pressure for a short time.

3. The process of claim 2 including the step of diluting the reactinggases with inert gases free from oxygen.

4. The process of claim 1 wherein the acid is in aqueous solution.

5. The process of claim 1 wherein the temperature is from about 580 C.to 700 C. and the acid compound is in aqueous solution and present in anamount of 20 yto 40% by weight 'of the reactants.

6. The process of claim 1 wherein the compound of the formula.'

Ri H

\C I -R'l 1151i 0H and the selected aci-d is blown as a tinely dividedre action mixture into a reaction zone.

7. The process of claim 6 wherein a h'ot, inert gas free from oxygenheats the finely divided reaction mixture in the reaction zone.

8. The process of claim 1 wherein the selected acid is in the gaseousstate.

9. The process for preparing acrylonitrile which comprises the step ofheating lactic acid nitrile with an acid selected from the groupconsisting of phosphoric acid, pyrophosphoric acid, metaandpolyphosphoric acids containing not more than six phosphorous atoms andhydrochloric acid at a temperature of from about 520- 700 C., the amountof acid by weight being from 1-90% of the total weight of the reactants.

References Cited in the file of this patent UNITED STATES PATENTS2,208,328 Lichty July 16, 1940 2,226,645 Thomas etal Dec. 3l, 19402,356,247 Kirk et al Aug. 22, 1944 2,452,672 Miller et al Nov. 2, 19482,469,701 Redmon May 10, 1949

1. A PROCESS FOR PREPARING A COMPOUND OF THE FORMULA: